1. Field of the Invention
The invention relates to an oxygen sensor deterioration detection apparatus and method, more particularly, to a deterioration detection apparatus suitable for detecting deterioration of an oxygen sensor whose element impedance exhibits a temperature characteristic.
2. Description of the Related Art
A related-art apparatus that detects an abnormality of an oxygen sensor on the basis of an element impedance of the oxygen sensor is disclosed in Japanese Patent Application Laid-Open Publication No. 2000-198635. In this related-art detection apparatus, the oxygen sensor is disposed in an exhaust passageway of an internal combustion engine. The oxygen sensor has a heater for heating a sensor element. In a normal service environment, the temperature of the sensor element is controlled at about 700° C. via the heater and heat from exhaust gas.
In the above-described detection apparatus, a characteristic of the element impedance of the sensor element is dependent on temperature. Therefore, if the heating of the sensor element is not appropriate, the element impedance exhibits a value that is different from the value exhibited when the heating is appropriate. Utilizing this characteristic, that is, on the basis of whether the element impedance exhibits a normal value in a normal service environment, the detection apparatus detects a broken wire related to the sensor, deterioration of the heater, a broken wire related to the heater, etc.
The detection apparatus assumes that the oxygen sensor has activated, at the elapse of a predetermined time following startup of the internal combustion engine. In that condition, the apparatus determines whether the oxygen sensor has an abnormality on the basis of the element impedance, on an assumption that the oxygen sensor has been heated to about 700° C.
In reality, however, the temperature of the oxygen sensor may sometimes be apart from the vicinity of 700° C. after the elapse of a predetermined time following startup of the internal combustion engine, even if the detection system is normal. In such a case, the value of the element impedance greatly deviates from the value that is normally exhibited when the temperature of the sensor element is at or around 700° C., due to the temperature characteristic of the sensor element. As a result, a false determination of an abnormality of the oxygen sensor may be made.